1. Field of the Invention
The present invention relates to a color image forming apparatus (such as a copying machine, a printer, or a facsimile (FAX)) using an electrophotography method.
2. Description of the Related Art
In recent years, a color image forming apparatus using an electrophotography method is widely used. Since the color image forming apparatus is required to provide precise color reproducibility and color stability, the color image forming apparatus is generally provided with a function for automatically executing image density control. In particular, due to variations in color caused by, for example, changes in the environment in which the color image forming apparatus is used and the history of use of various consumable items, it is necessary to periodically execute the image density control for stabilizing the color at all times.
In an example of the image density control, a plurality of test toner images (patches), formed on an image bearing member while changing an image-formation condition, are detected with an optical image density detector, disposed in the image forming apparatus. In this case, a detection result of the optical image density detector is converted to a toner adhesion amount, to set suitable image-formation conditions on the basis of a conversion result. Here, examples of image-formation conditions include dynamic conditions (such as charging voltage, exposure strength, and development voltage) and corrections (adjustments) of a conversion condition table used when forming a half tone image. Here, when the toner adhesion amount is not a toner amount (g), the toner adhesion amount may be any amount equivalent to the toner amount (g) that can be determined by a printer body.
Here, the operation of the optical image density detector will be described in more detail. First, basically, a patch or an image bearing member is irradiated with light by a light-emitting element, and light reflected from the patch or the image bearing member is received by a photodetector. On the basis of a result obtained when the light is received by the photodetector, the toner adhesion amount of the patch is calculated. Here, for stabilizing detection precision, it is important that the quantity of light emitted from the light-emitting element be set at a suitable value. When the light-emission quantity is too large, the quantity of light reflected from the patch or the image bearing member becomes too large. This causes an output of the photodetector to be fixed at an upper limit. As a result, the toner adhesion amount cannot be precisely calculated. On the other hand, when the light-emission quantity is too small, the quantity of light reflected from the patch or the image bearing member becomes too small. In addition, a change in output of the photodetector becomes small with respect to a change in the toner adhesion amount of the patch. When this is converted to the toner adhesion amount, an error becomes large. Further, the output of the photodetector changes with, for example, a change in reflectivity (caused by deterioration of the image bearing member (which is a detection surface) with time), staining of the image density detector with time, or a lot variation of structural components of the image density detector. From this viewpoint, it is important that the light-emission quantity be set at a suitable value.
On the basis of such a background, in general, sensor characteristics are corrected before detecting a toner adhesion amount (that is, before controlling image density). Practical forms are discussed in, for example, Japanese Patent Laid-Open Nos. 2002-229279 and 2000-13190. Here, the term “correction” refers to adjustment of a toner-adhesion-amount sensor output to a constant/substantially constant value by adjusting the light-emission quantity of a sensor light-emitting element (LED, etc).
In controlling the image density, in general, first, the light-emission quantity is adjusted. Then, after obtaining an output VB of the photodetector when there is no adhesion of toner, the image bearing member is rotated. Then, patches are formed to obtain an output VP of the photodetector. The quantity of light emitted from the light-emitting element is generally made equal to a light-emission quantity obtained on the basis of the outputs VB and VP because it takes time for an output of light to be stabilized. In addition, for adjusting light quantity, it is necessary to form a solid patch on the image bearing member. Further, the solid patch needs to be completely eliminate. This is because, if the output VB is obtained when the solid patch is not sufficiently eliminated, the toner amount cannot be precisely calculated. Here, the term “completely” means “sufficiently” in detecting the density, so that the solid patch is not actually eliminated completely.
According to the above-described background, ordinarily, as shown in FIG. 27, the image density is controlled after increasing the number of rotations of the image bearing member and removing toner.
However, as a consequence, in addition to formation/detection of a patch (indicated by reference numeral 2601 in FIG. 27), for example, cleaning of the intermediate transfer belt is performed many times due to removal of the solid patch. Therefore, processing time is increased.
Although it is known that there is a risk that the solid patch cannot be completely eliminated, reducing the number of rotations of the image bearing member and omitting the removal of the toner make it possible to reduce an image density controlling time. However, in this case, the precision with which the image density is controlled is reduced.